It is well known to add certain resins to paper, usually during the paper-making process, to improve dry-strength of the resultant paper. It is also well known to add certain resins to paper to improve wet-strength of the resultant paper. It is also well known that certain additives increase both a paper's wet-strength and and dry-strength. However, it is not always desirable that paper with increased dry-strength also exhibit an increased wet-strength since increasing a paper's wet-strength makes paper more difficult to repulp. If it is difficult for a paper product to be repulped, the papermaker will find it difficult to reprocess the material thereby increasing the amount of unusable waste associated with the papermaking process.
Many polymers that improve a paper's dry-strength are anionic under normal papermaking conditions, e.g., sodium carboxymethylcellulose, carboxymethyl guar, and copolymers of acrylamide and acrylic acid or sodium acrylate. A cationic compound may be used to retain these anionic substances on the pulp fibers which are themselves anionic by nature. These cationic compounds may be wet-strength agents such as polyaminoamide-epichlorohydrin resins, highly cationic polymers such as poly(diallyldimethylammonium chloride), polymers of dimethylamine and epichlorohydrin, modified starches, aluminum compounds, etc.
In U.S. Pat. No. 5,338,406 to Smith, a dry-strength system for a "water-soluble, linear, high molecular weight, low charge density cationic polymer having a reduced specific viscosity greater than two deciliters per gram (&gt;2 dl/g) and a charge density of 0.2 to 4 milliequivalents per gram" with "at least one water-soluble, anionic polymer having a charge density less than 5 meq/g" is disclosed. The polyelectrolyte complex of Smith is useful as an additive for providing dry-strength to all types of paper, particularly for those papers which are produced using unbleached pulp.
In U.S. Pat. No. 5,338,407 to Dasgupta, a process for enhancement of paper dry-strength without reducing its softness is disclosed. The process comprises adding a mixture of an anionic carboxymethyl guar, carboxymethyl bean gum or carboxymethyl hydroxyethyl guar with various cationic additives to a bleached pulp furnish. The cationic additive may be a polyamidoamine-epichlorohydrin resin. If the cationic additive is a wet-strength resin, the paper's dry-strength is enhanced without reducing its softness. Additionally, the wet-strength of the paper is increased.
In Canadian Patent No. 1,110,019, "a process for manufacturing paper having improved dry-strength which comprises mixing an essentially alum-free pulp slurry with a water-soluble cationic polymer and subsequently adding a water-soluble anionic polymer to the essentially alum-free pulp slurry" is disclosed.
In addition to the above, polyamidoamine-epichlorohydrin resins have been used extensively as wet-strength agents for paper. Typically, these resins are prepared in a two-step process. In a first step, a polyamidoamine prepolymer is prepared from a diacid (e.g. adipic acid) and a polyamine (e.g. diethylenetriamine). In a second step, the polyamidoamine prepolymer is reacted with epichlorohydrin in an amount equal to or greater than the amount of secondary amine groups in the prepolymer. A small amount of epichlorohydrin reacts to effect branching of the prepolymer, accompanied by an increase in molecular weight. However, a majority of the epichlorohydrin reacts with the prepolymer to give reactive functional groups, specifically, either aminochlorohydrin or azetidinium. It is well known to those skilled in the art of papermaking that the above-described polyamidoamine-epichlorohydrin resins may be used in combination with anionic acrylamides or anionic cellulose derivatives. However, papers containing these combinations exhibit increased wet-strength as well as increased dry-strength, thereby making papers containing these combinations difficult to repulp.